Cryogenically cooled superconductor rf head coil array and head-only magnetic resonance imaging (mri) system using same

ABSTRACT

A cryogenically-cooled superconducting RF head-coil array which may be used in whole-body MRI scanners and/or in dedicated, head-only MRI systems. An RF head-coil array module may comprise a vacuum thermal isolation housing comprising a double wall hermetically sealed jacket that (i) encloses a hermetically sealed interior space under a vacuum condition, and (ii) substantially encloses an interior chamber region that is separate from the hermetically sealed interior space and is configured to be evacuated to a vacuum condition. A plurality of superconductor radiofrequency coils are disposed in the interior chamber region, and each radiofrequency coil is configured for at least one of generating and receiving a radiofrequency signal for at least one of magnetic resonance imaging and magnetic resonance spectroscopy. At least one thermal sink member may be disposed in the interior chamber region and in thermal contact with the superconductor radiofrequency coils. A port is configured for cryogenically cooling the thermal sink members.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/171,074, filed Apr. 20, 2009, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates generally to magnetic resonance imagingand spectroscopy, and, more particularly, to magnetic resonance imagingand spectroscopy apparatus employing superconductor components, and tomethods for manufacturing such apparatus.

BACKGROUND

Magnetic Resonance Imaging (MRI) technology is commonly used today inlarger medical institutions worldwide, and has led to significant andunique benefits in the practice of medicine. While MRI has beendeveloped as a well-established diagnostic tool for imaging structureand anatomy, it has also been developed for imaging functionalactivities and other biophysical and biochemical characteristics orprocesses (e.g., blood flow, metabolites/metabolism, diffusion), some ofthese magnetic resonance (MR) imaging techniques being known asfunctional MRI, spectroscopic MRI or Magnetic Resonance SpectroscopicImaging (MRSI), diffusion weighted imaging (DWI), and diffusion tensorimaging (DTI). These magnetic resonance imaging techniques have broadclinical and research applications in addition to their medicaldiagnostic value for identifying and assessing pathology and determiningthe state of health of the tissue examined.

During a typical MRI examination, a patient's body (or a sample object)is placed within the examination region and is supported by a patientsupport in an MRI scanner where a substantially constant and uniformprimary (main) magnetic field is provided by a primary (main) magnet.The magnetic field aligns the nuclear magnetization of precessing atomssuch as hydrogen (protons) in the body. A gradient coil assembly withinthe magnet creates a small variation of the magnetic field in a givenlocation, thus providing resonance frequency encoding in the imagingregion. A radio frequency (RF) coil is selectively driven under computercontrol according to a pulse sequence to generate in the patient atemporary oscillating transverse magnetization signal that is detectedby the RF coil and that, by computer processing, may be mapped tospatially localized regions of the patient, thus providing an image ofthe region-of-interest under examination.

In a common MRI configuration, the static main magnetic field istypically produced by a solenoid magnet apparatus, and a patientplatform is disposed in the cylindrical space bounded by the solenoidwindings (i.e. the main magnet bore). The windings of the main field aretypically implemented as a low temperature superconductor (LTS)material, and are super-cooled with liquid helium in order to reduceresistance, and, therefore, to minimize the amount of heat generated andthe amount of power necessary to create and maintain the main field. Themajority of existing LTS superconducting MRI magnets are made of aniobium-titanium (NbTi) and/or Nb₃Sn material which is cooled with acryostat to a temperature of 4.2 K.

As is known to those skilled in the art, the magnetic field gradientcoils generally are configured to selectively provide linear magneticfield gradients along each of three principal Cartesian axes in space(one of these axes being the direction of the main magnetic field), sothat the magnitude of the magnetic field varies with location inside theexamination region, and characteristics of the magnetic resonancesignals from different locations within the region of interest, such asthe frequency and phase of the signals, are encoded according toposition within the region (thus providing for spatial localization).Typically, the gradient fields are created by current passing throughcoiled saddle or solenoid windings, which are affixed to cylindersconcentric with and fitted within a larger cylinder containing thewindings of the main magnetic field. Unlike the main magnetic field, thecoils used to create the gradient fields typically are common roomtemperature copper windings. The gradient strength and field linearityare of fundamental importance both to the accuracy of the details of theimage produced and to the information on tissue chemistry (e.g., inMRSI).

Since MRI's inception, there has been a relentless pursuit for improvingMRI quality and capabilities, such as by providing higher spatialresolution, higher spectral resolution (e.g., for MRSI), highercontrast, and faster acquisition speed. For example, increased imaging(acquisition) speed is desired to minimize imaging blurring caused bytemporal variations in the imaged region during image acquisition, suchas variations due to patient movement, natural anatomical and/orfunctional movements (e.g., heart beat, respiration, blood flow), and/ornatural biochemical variations (e.g., caused by metabolism during MRSI).Similarly, for example, because in spectroscopic MRI the pulse sequencefor acquiring data encodes spectral information in addition to spatialinformation, minimizing the time required for acquiring sufficientspectral and spatial information to provide desired spectral resolutionand spatial localization is particularly important for improving theclinical practicality and utility of spectroscopic MRI.

Several factors contribute to better MRI image quality in terms of highcontrast, resolution, and acquisition speed. An important parameterimpacting image quality and acquisition speed is the signal-to-noiseratio (SNR). Increasing SNR by increasing the signal before thepreamplifier of the MRI system is important in terms of increasing thequality of the image. One way to improve SNR is to increase the magneticfield strength of the magnet as the SNR is proportional to the magnitudeof the magnetic field. In clinical applications, however, MRI has aceiling on the field strength of the magnet (the US FDA's currentceiling is 3 T (Tesla)). Other ways of improving the SNR involve, wherepossible, reducing sample noise by reducing the field-of-view (wherepossible), decreasing the distance between the sample and the RF coils,and/or reducing RF coil noise.

Despite the relentless efforts and many advancements for improving MRI,there is nevertheless a continuing need for yet further improvements inMRI, such as for providing greater contrast, improved SNR, higheracquisition speeds, higher spatial and temporal resolution, and/orhigher spectral resolution.

Additionally, a significant factor affecting further use of MRItechnology is the high cost associated with high magnetic field systems,both for purchase and maintenance. Thus, it would be advantageous toprovide a high quality MRI imaging system that is capable of beingmanufactured and/or maintained at reasonable cost, permitting MRItechnology to be more widely used.

SUMMARY OF INVENTION

Various embodiments of the present invention provide a cryogenicallycooled superconducting RF head-coil array which may be used inwhole-body MRI scanners and/or in dedicated, head-only MRI systems (alsoreferred to herein as “head-dedicated MRI systems,” “head-only MRIsystems,” or the like). Some embodiments of the invention provide ahead-dedicated MRI system and, more particularly, various embodimentsprovide a superconducting main magnet for a head-dedicated MRI systemwhich, in some embodiments, further comprises a cryogenically-cooledsuperconducting RF head-coil array according to embodiments of thepresent invention.

In accordance with some embodiments, a superconducting radiofrequencycoil array module configured for cryogenic cooling comprises: a vacuumthermal isolation housing comprising a double wall hermetically sealedjacket that (i) encloses a hermetically sealed interior space under avacuum condition, and (ii) substantially encloses an interior chamberregion that is separate from the hermetically sealed interior space andis configured to be evacuated to a vacuum condition; a plurality ofsuperconductor radiofrequency coils disposed in said interior chamberregion and configured, each radiofrequency coil configured for at leastone of generating and receiving a radiofrequency signal for at least oneof magnetic resonance imaging and magnetic resonance spectroscopy; atleast one thermal sink member disposed in said interior chamber regionand in thermal contact with the superconductor radiofrequency coils; anda port configured for cryogenically cooling at least the thermal sinkmember. The port may be coupled to a cryocooler that is thermallycoupled to the at least one thermal sink member.

In some embodiments, each radiofrequency coil is in direct thermalcontact with a respective one of the thermal sink members that are eachin direct thermal contact with another of the thermal sink members thatis in thermal contact with the cryocooler.

The radiofrequency coils may comprise at least eight radiofrequencycoils that are azimuthally displaced about a common longitudinal axis ata substantially common displacement along the longitudinal axis, and areconfigured for imaging a region surrounded by the radiofrequency coils.Each of the radiofrequency coils may be configured to receive and nottransmit radiofrequency signals.

The vacuum thermal isolation housing and radiofrequency coils may bedimensioned and configured for head imaging and not whole body imaging.In some embodiments, the radiofrequency coil array module is dimensionedand configured for use in a head-only magnetic resonance imaging systemthat comprises a main electromagnet system comprising: a first andsecond set of high temperature superconductor coils which are configuredto be coaxial relative to a common longitudinal axis; wherein the firstcoil set includes at least two coils having an inner radius and disposedin a first region of a length along the common axis to cover a head andneck of a human body, and the second coil set includes at least one coilhaving an inner radius and disposed in a second region of a length alongthe common axis to cover a portion of a human torso; and wherein thefirst and second coils are configured to provide a uniform magneticfield in the first region to provide for imaging a region of interest ofthe individual's head when positioned within the first region.

It will be appreciated by those skilled in the art that the foregoingbrief description and the following detailed description are exemplaryand explanatory of the present invention, but are not intended to berestrictive thereof or limiting of the advantages which can be achievedby this invention. Additionally, it is understood that the foregoingsummary of the invention is representative of some embodiments of theinvention, and is neither representative nor inclusive of all subjectmatter and embodiments within the scope of the present invention. Thus,the accompanying drawings, referred to herein and constituting a parthereof, illustrate embodiments of this invention, and, together with thedetailed description, serve to explain principles of embodiments of theinvention. Aspects, features, and advantages of embodiments of theinvention, both as to structure and operation, will be understood andwill become more readily apparent when the invention is considered inthe light of the following description made in conjunction with theaccompanying drawings, in which like reference numerals designate thesame or similar parts throughout the various figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects, features, and advantages of embodiments of the invention, bothas to structure and operation, will be understood and will become morereadily apparent when the invention is considered in the light of thefollowing description made in conjunction with the accompanyingdrawings, in which like reference numerals designate the same or similarparts throughout the various figures, and wherein:

FIGS. 1A and 1B schematically depict orthogonal views of an illustrativecryogenically cooled superconducting RF head coil array, in accordancewith some embodiments of the present invention;

FIG. 2 schematically illustrates wall(s) of the vacuum chamber depictedin FIG. 1A being implemented as a double-walled glass Dewar, inaccordance with some embodiments of the present invention;

FIG. 3 schematically depicts an illustrative cross-sectional view alongthe longitudinal axis of a superconductor RF head coil arraycorresponding to embodiments depicted in FIGS. 1A and 1B with the vacuumchamber comprising a Dewar 1 according to various embodimentsrepresented by FIG. 2, in accordance with some embodiments of thepresent invention;

FIGS. 4A and 4B, depict an illustrative alternative implementation of asuperconductor RF head coil array (module), in accordance with someembodiments of the present invention;

FIG. 5 schematically depicts a cross section of an illustrative MRIsystem, in accordance with some embodiments of the present invention;

FIG. 6 schematically depicts an illustrative RF head coil array thatincludes thermal radiation screening, in accordance with someembodiments of the present invention;

FIG. 7 schematically depicts a cross-sectional view of a superconductingmain magnet of a head-only MRI system, in accordance with someembodiments of the present invention;

FIG. 8 depicts with reference to the z-r plane a coil configuration of asuperconducting main magnet system, in accordance with some embodimentsof the present invention;

FIG. 9 depicts a normalized current distribution for the main magnetcoil arrangement corresponding to the illustrative embodiment of FIGS. 7and 8, in accordance with some embodiments of the present invention;

FIG. 10 is an illustrative coil pattern (depicted in the z-r plane, withunits normalized to meters) of a 3 T head magnetic resonance imagingscanner, in accordance with various embodiments of the presentinvention;

FIG. 11 is a plot showing the magnetic field distribution for theillustrative embodiment depicted in FIG. 10, in accordance with someembodiments of the present invention; and

FIG. 12 shows the fringe fields of one Gauss (1 G), three Gauss (3 G)and five Gauss (5 G) lines for the field distribution of FIG. 11, inaccordance with an illustrative embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The ensuing description discloses (i) various embodiments of acryogenically cooled superconducting RF head-coil array which may beused in whole-body MRI scanners and/or in dedicated, head-only MRIsystems (also referred to herein as “head-dedicated MRI systems,”“head-only MRI systems,” or the like) and (ii) various embodiments of ahead-dedicated MRI system and, more particularly, various embodiments ofa superconducting main magnet for a head-dedicated MRI system which, insome embodiments, further comprises a cryogenically-cooledsuperconducting RF head-coil array according to embodiments of thepresent invention.

More specifically, as will be further understood by those skilled in theart in view of the ensuing description, a cryogenically-cooledsuperconducting RF head-coil array coil according to various embodimentsof the present invention may be implemented in myriad magnetic resonanceimaging and spectroscopy systems, such as systems employing conventionalcopper gradient coils, systems employing superconducting gradient coils(e.g., such as disclosed in U.S. patent application Ser. No. 12/416,606,filed April 1, 2009, and in Provisional Application No. 61/170,135,filed Apr. 17, 2009, each of which is hereby incorporated by referencein its entirety), whole body systems, dedicated head-only systems,systems with a vertically or horizontally oriented main magnetic field,open or closed systems, etc. Similarly, as will be further understood bythose skilled in the art in view of the ensuing description, ahead-dedicated MRI system employing a superconducting main magnetaccording to various embodiments of the present invention may beimplemented in myriad magnetic resonance imaging and spectroscopysystems, such as systems employing conventional copper gradient coils,systems employing superconducting gradient coils (e.g., such asdisclosed in U.S. patent application Ser. No. 12/416,606, filed Apr. 1,2009, and in Provisional Application No. 61/170,135, filed Apr. 17,2009, each of which is hereby incorporated by reference in itsentirety), systems employing conventional (e.g., copper) head coils orcoil arrays, and/or systems employing a superconducting RF head coilarray (e.g., according to superconducting RF head-coil embodimentsdescribed herein), etc. Similarly, it will also be understood by thoseskilled in the art that while various portions of the ensuingdescription may be set forth in the context of an MRI system that may beused for structural examination of a patient, various embodiments of thepresent invention may be employed in connection with magnetic resonance(MR) systems operated and/or configured for other modalities, such asfunctional MRI, diffusion weighted and/or diffusion tensor MRI, MRspectroscopy and/or spectroscopic imaging, etc. Additionally, as usedherein, MRI includes and embraces magnetic resonance spectroscopicimaging, diffusion tensor imaging (DTI), as well as any other imagingmodality based on nuclear magnetic resonance.

FIGS. 1A and 1B schematically depict orthogonal views of an illustrativecryogenically cooled superconducting RF head coil array 10, inaccordance with some embodiments of the present invention. (Forconvenience and ease of reference and additional clarity of exposition,orthogonal x, y, z coordinates are depicted as a reference frame.) Morespecifically, FIG. 1A is a cross-sectional view in the x-y planeindicated by reference IA-IA′ in FIG. 1B, and illustrates aconfiguration of eight superconducting RF coils 3 a-3 h (also referredto herein collectively as superconductor RF coils 3 or RF coil array 3)each disposed in thermal contact with a respective one of eight thermalconductors 5 a-5 h (e.g., non-metallic high thermal conductivitymaterials, such as high thermal conductivity ceramic, such as sapphireor alumina), with the RF coils 3 a-3 h and thermal conductors 5 a-5 hbeing disposed within a sealed vacuum chamber having vacuum chamberwall(s) 2.

FIG. 1B is a side view along the longitudinal axis (i.e., z axis) viewedfrom the direction indicated by reference IB in FIG. 1A, and illustratescomponents comprising the cooling system of superconducting RF head coilarray 10, the cooling system including thermal conductor 15 (e.g.,non-metallic high thermal conductivity materials, such as high thermalconductivity ceramic, such as sapphire or alumina) in thermal contactwith each of thermal conductors 5 a-5 h, cold head 9 in thermal contactwith thermal conductor (sink) 15, and cryocooler 7 configured formaintaining the cold head 9 at a desired cryogenic temperature. Forclarity of exposition, however, FIG. 1B does not show (i) the vacuumchamber comprising vacuum chamber wall(s) 2, (ii) coils 3 b and 3 d, and(iii) thermal conductors 5 b and 5 d (as will be further understood fromthe ensuing description (e.g., in connection with FIG. 3), FIG. 1B alsodoes not show a vacuum chamber portion into which cryocooler 7 ismounted).

Accordingly, in the configuration of the superconducting RF head coilarray 10 depicted in FIGS. 1A and 1B, coils 3 a-3 h are in vacuum andcooled by the thermal conductors 5 a-5 b, which conduct heat away fromthe coils to the thermal conductor/sink 15, which is thermally coupledwith a cryogenic cooler 7. As will be understood by those skilled in theart, in some embodiments (e.g., low main magnetic field implementations,such as less than 3 T, or less than 1.5 T, etc.) small amounts of metal,such as copper, may be used for thermal conductor/sink 15 and/orpossibly thermal conductors 5 a-5 h. In some embodiments, thermalconductors 5 a-5 h may be integrally formed with thermal conductor/sink15, whereas in some embodiments, one or more of thermal conductors 5 a-5h are distinct members that are mechanically joined (e.g., using epoxy,etc.) to thermal conductor/sink 15 to provide a good thermal conductiontherebetween. In various embodiments, the coils 3 a-3 h may be cooled toa temperature in the range of about 4K to 100K, and more particularly,to a temperature below the critical temperature of the superconductingmaterial (e.g., in some embodiments, below the critical temperature of ahigh temperature superconductor (HTS) material used for the RF coils 3a-3 h).

More particularly, in accordance with various embodiments of the presentinvention, each of RF coil elements 3 a-3 h is implemented as a hightemperature superconductor (HTS), such as YBCO and/or BSCCO, etc. (e.g.,using an HTS thin film or HTS tape), though a low temperaturesuperconductor (LTS) may be used in various embodiments. For example, insome embodiments, each of RF coil elements 3 a-3 h is an HTS thin filmspiral coil and/or an HTS thin film spiral-interdigitated coil on asubstrate such as sapphire or lanthanum aluminate. The design andfabrication of such coils is further described in and/or may be furtherunderstood in view of, for example, Ma et al., “Superconducting RF Coilsfor Clinical MR Imaging at Low Field,” Academic Radiology, vol. 10, no.9, September 2003, pp. 978-987; Gao et al., “Simulation of theSensitivity of HTS Coil and Coil Array for Head Imaging,” ISMRM-2003,no. 1412; Fang et al., “Design of Superconducting MRI Surface Coil byUsing Method of Moment,” IEEE Trans. on Applied Superconductivity, vol.12, no. 2, pp. 1823-1827 (2002); and Miller et al., “Performance of aHigh Temperature Superconducting Probe for In Vivo Microscopy at 2.0 T,”Magnetic Resonance in Medicine, 41:72-79 (1999), each of which isincorporated by reference herein in its entirety. Accordingly, in someembodiments, superconducting RF head coil array 10 is implemented as anHTS thin film RF head coil array.

As depicted in FIG. 2, in accordance with some embodiments of thepresent invention, vacuum chamber comprising wall(s) 2 may comprise adouble-walled Dewar 1 made of glass and/or other non-conductive,mechanically strong material(s), such as G10, RF4, plastic, and/orceramic. More specifically, FIG. 2 schematically illustrates wall(s) 2of the vacuum chamber depicted in FIG. 1A being implemented as adouble-walled glass Dewar 1, in accordance with some embodiments of thepresent invention. It will be understood that the dimensions and shapeof a cryogenically cooled superconducting RF head-coil array module maybe modified according to various implementations of the presentinvention. In accordance with some implementations, FIG. 2 schematicallyillustrates a glass dewar portion 1 of a cryogenically cooledsuperconducting RF head-coil array module that may be used, for example,in a magnetic resonance imaging system dedicated to head imaging,wherein the glass dewar components may have the following approximatedimensions, provided merely by way of example and for additional clarityof exposition: cylinder 60 has an inner diameter, outer diameter, andaxial length of 230 mm, 236 mm, and 254 mm, respectively; cylinder 62has an inner diameter, outer diameter, and axial length of 246 mm, 252mm, and 254 mm, respectively; cylinder 64 has an inner diameter, outerdiameter, and axial length of 280 mm, 286 mm, and 312 mm, respectively;cylinder 66 has an inner diameter, outer diameter, and axial length of296 mm, 302 mm, and 330 mm, respectively; inner bottom plate(circular/cylindrical) 74 has a diameter of 236 mm and a thickness of12.7 mm; outer bottom plate (circular/cylindrical) 76 has a diameter of252 mm and a thickness of 12.7 mm; ring (annular) 66 has an innerdiameter, outer diameter, and thickness (axial) of 246 mm, 286 mm, and12.7 mm, respectively; ring (annular) 68 has an inner diameter, outerdiameter, and thickness (axial) of 230 mm, 302 mm, and 12.7 mm,respectively; and ring (annular) 72 has an inner diameter, outerdiameter, and thickness (axial) of 280 mm, 302 mm, and 12.7 mm,respectively. Also shown are two of eight small spacer disks 78, havingan approximate diameter of 5 mm as well as a height that provides for agap of about 5 mm between the inner bottom plate 74 and outer bottomplate 76. In this illustrative embodiment, a plug 70 seals off astandard vacuum port in ring 68 through which the intra-dewar cavity isevacuated.

It will be understood that double-walled Dewar 1 may be constructed, ina variety of ways, as a continuous, hermetically sealed glass housingenclosing an interior chamber (or cavity) 4 in which at least a lowvacuum condition and, in accordance with some embodiments, preferably atleast a high vacuum condition (e.g., about 10⁻⁶ Torr or lower pressure)is maintained. For example, in accordance with some embodiments,double-walled Dewar 1 may be manufactured as follows: (i) forming twogenerally cylindrical (e.g., but hexagonal in cross-section transverseto the longitudinal/cylindrical access) double-walled structures eachhaving a generally U-shaped wall cross-section, the first correspondingto continuous glass wall portion 1 a (comprising cylinders 60 and 66,ring 68 and plate 74) and the second corresponding to continuous wallportion 1 b (comprising cylinders 62 and 64, ring 66, and plate 76),(ii) fitting the generally cylindrical continuous glass wall portion 1 binto the annular space of generally cylindrical continuous glass wallportion 1 a, possibly using glass spacers therebetween (e.g., identifiedin FIG. 2 as disks 78), and (iii) glass-bonding, fusing, or otherwisesealing the open end between 1 a and 1 b (i.e., the end that is latersealably mounted to stainless steel chamber 8, further described belowin connection with FIG. 3), (e.g., by bonding, fusing, or otherwisesealing ring 72 to the open end) to hermetically seal cavity 4 underhigh vacuum, and (iv) pumping the cavity 4 to a high vacuum through thedepicted standard vacuum port, which is hermetically sealed (e.g., usingcap 70) after pumping to the desired vacuum pressure. It may beappreciated that the vacuum sealing step may be performed in myriadways. For example, portions 1 a and 1 b may be joined and sealed to eachother within a vacuum chamber, or, as described, the ends of 1 a and 1 bmay be fused to each other except for a small region that is used as avacuum pumping port and that is sealed after pumping the cavity to highvacuum therethrough. In various embodiments, double-walled Dewar 1 maybe implemented in accordance with, or similar to, the hermeticallysealed double-walled structures (and vacuum thermal isolation housing)described in U.S. application Ser. No. 12/212,122, filed Sep. 17, 2008,and in U.S. application Ser. No. 12/212,147, filed Sep. 17, 2008, eachof which is herein incorporated by reference in its entirety.

FIG. 3 schematically depicts an illustrative cross-sectional view alongthe longitudinal axis of a superconductor (e.g., HTS) RF head coil arraycorresponding to embodiments depicted in FIGS. 1A and 1B with the vacuumchamber comprising a Dewar 1 according to various embodimentsrepresented by FIG. 2. As shown, Dewar 1 is sealably joined to adouble-walled stainless steel chamber 8 that includes a flange to whichcryocooler 7 is sealably mounted. In various embodiments, double-walledstainless steel chamber 8 is hermetically sealed, enclosing an interiorchamber (or cavity) 12 in which at least a low vacuum condition and, inaccordance with some embodiments, preferably at least a high vacuumcondition (e.g., about 10⁻⁶ Torr or lower pressure) is maintained. Byway of example, the joint between the hermetically sealed double-walledDewar 1 (e.g., glass) and the stainless steel chamber may be formed byepoxy bonding, welding, or other hermetically sealed flange connection,providing a sufficient seal to maintain at least a low vacuum condition(e.g., about 10⁻² to about 10⁻⁵ Torr) in the interior chamber portion 6that houses the superconducting RF coils 3 and thermal conductors 5(i.e., 5 a-5 h) and 15. Also by way of example, the vacuum seal betweencryocooler 7 and the flange of stainless steel chamber 8 may be providedby an O-ring or other sealing mechanism (e.g., metal gasket/knife-edgeconnection) to, similarly, maintain the at least low vacuum condition inthe interior chamber portion 6 that houses the RF coils 3 and thermalconductors 5 and 15. Those skilled in the art understand, however, thatchamber 8 may be made of materials other than stainless steel, e.g.,aluminum or other metallic or other non-metallic material, such asglass, ceramic, plastics, or combination of these materials, and suchother materials may be appropriately joined to Dewar 1 and cryocooler 7.

In various embodiments, cryocooler 7 may be implemented as any ofvarious single stage or multi-stage cryocoolers, such as, for example, aGifford McMahon (GM) cryocooler, a pulse tube (PT) cooler, aJoule-Thomson (JT) cooler, a Stirling cooler, or other cryocooler. Invarious alternative embodiments, the superconductor RF head coil array10 may be configured for cooling such that coils 3 are cooled by acryogen, such as liquid helium and liquid nitrogen.

It is understood that while not shown in the drawings, a cryogenicallycooled superconductor RF coil array (e.g., array 10) in accordance withvarious embodiments of the present invention includes at least oneelectrical feedthrough (e.g., through chamber 8) to provide for couplingelectrical signals into and/or out of the array (e.g., for the RF coils,for controlling and/or monitoring any sensors (e.g., pressure and/ortemperature, etc.) that may be provided in the module). Additionally, itwill be understood that at least a portion of receiver and/or, ifapplicable, transmitter circuitry (e.g., amplifiers and/or filtersand/or appropriate matching and/or decoupling circuitry) for each of theRF coils may be provided within the vacuum chamber; for example, it maybe disposed on and in thermal contact with thermal conductors 5 a-5 h,wherein such cooling may provide for improving noise properties and/orfor using superconducting components for at least a portion of suchcircuitry.

As understood in view of the foregoing description, in accordance withvarious embodiments of the present invention, superconducting RF headcoil array 10 is implemented as a receive-only array, with an RFtransmitter being implemented as a separate RF coil (not shown), whichin various embodiments may be a conventional (e.g., non-superconducting,such as a conventional copper RF coil) RF transmitter coil or asuperconducting RF transmitting coil. Such a separate transmitter coilmay be configured external to the vacuum chamber comprising wall(s) 2(e.g., external to Dewar 1) or, in some embodiments, within the vacuumchamber comprising wall(s) 2 (e.g., within Dewar 1). For instance, inthe case that an RF transmission coil is implemented as one or moresuperconducting RF transmission coils (e.g., a high temperaturesuperconductor (HTS) RF transmitter) that are separate from the RFreceiver coils, then, in some embodiments, such one or moresuperconducting RF transmission coils may be disposed in thermal contactwith one or more of thermal conductors 5 a-5 h.

In some embodiments, superconducting RF head coil array 10 may beimplemented as a transmit and receive coil array (a transceiver array),with each of one or more of the superconducting RF coils 3 a-3 h beingused for both transmission and reception of RF signals.

In accordance with various embodiments of the present invention, one ormore of the superconducting RF coil elements 3 a-3 h may be implementedas a multiple resonance RF coil element (e.g., comprising two or morereceiving coils having different resonant frequencies, such as fordetecting sodium and hydrogen resonances at a given magnetic field(e.g., at 3 Tesla (T)). In some embodiments, two or more different onesof superconducting RF coil elements 3 a-3 h may be designed to havedifferent resonant frequencies; for example, RF coil elements 3 a, 3 c,3 e, and 3 g may be tuned to a first resonant frequency (e.g., that ofhydrogen nuclei at 3 T) and RF coil elements 3 b, 3 d, 3 f, and 3 h maybe tuned to a second resonant frequency (e.g., that of sodium nuclei at3 T). As such, a superconducting RF head coil array in accordance withvarious embodiments of the present invention may be used for acquiringmagnetic resonance signals from different types of nuclei in asimultaneous or time-multiplexed manner.

It is further understood that while the hereinabove described figuresdepict an illustrative embodiment of a superconducting RF head coilarray having eight RF receiving channels (e.g., eight receiver coils),alternative embodiments of the present invention may comprisesuperconducting RF head coil arrays having less or more than eightsuperconducting RF receiving channels (e.g., less or more than eight RFreceiver.

Additionally, as indicated above, it is understood that according tosome embodiments of the present invention, a cryogenically-cooledsuperconducting RF head-coil array coil according to various embodimentsof the present invention may be implemented in a magnetic resonanceimaging system that employs superconducting gradient coils such as thosedisclosed in U.S. patent application Ser. No. 12/416,606, filed Apr. 1,2009, and in Provisional Application No. 61/170,135, filed Apr. 17,2009, each of which is hereby incorporated by reference in its entirety.In some embodiments, one or more of the superconducting gradient coilsmay be disposed within the same vacuum chamber as the superconducting RFcoils (e.g., the gradient coils may be in thermal contact with thesurfaces of thermal conductors 5 a-5 h that are opposite the surfaces incontact with coils 3 a-3 h).

Referring now to FIGS. 4A and 4B, there is shown an illustrativealternative implementation of a superconductor RF head coil array(module), in accordance with some embodiments of the present invention.More specifically, FIG. 4A schematically depicts a cross-sectional viewin a plane containing the longitudinal axis, similar to thecross-sectional view depicted with respect to the embodiment of FIG. 3(e.g., viewing an x-z plane cross-section, using a coordinate systemoriented similarly to that for the embodiment of FIGS. 1A, 1B, 2 and 3),while FIG. 4B generally depicts a plan or end-on view, viewed from theleft-hand side of FIG. 4A, but showing a cut-away or cross-section ofstainless steel chamber 8 to reveal the portion of cryocooler 7 withinthe chamber 8. As may be appreciated, because the embodiment depicted inFIGS. 4A and 4B is similar to that of FIGS. 1A, 1B, 2 and 3, forconvenience and ease of reference, identical reference numerals havebeen used to identify corresponding or similar elements. As may also beunderstood, a difference between the embodiment depicted in FIGS. 1B, 2and 3 and the embodiment depicted in FIG. 4A and 4B is that the formerembodiment is configured such that the end disposed near the cryocooleris closed, whereas the dewar 1 and chamber 8 (sealably connected via,e.g., epoxy bond/sealing 16) of the latter embodiment are configured toprovide for the end disposed near the cryocooler being open. Similarly,in connection with the open-ended design of FIGS. 4A and 4B, a thermalconductive ring 25 (cylindrical ring) is thermally coupled to eachthermal conductor 5 a-h (5 a and 5 e shown in FIG. 4A) and to cryocooler7, which is sealably mounted (e.g., via an O-ring sealed flange 19) tochamber 8.

As will be understood by those skilled in the art, a generallycylindrically shaped RF head coil array module such as depicted in theforegoing described embodiments may be well suited for use, for example,in an MRI system that employs a cylindrical, solenoid main magnetstructure that generates a substantially uniform, horizontal magneticfield. For example, such an MRI system is schematically depicted in FIG.5 in longitudinal cross section, and includes cylindrical main magnet 17having a bore in which a superconductor RF head coil array (module) 10corresponding to that of FIGS. 4A and 4B is disposed, and which alsoincludes gradient coil(s) 13. It will be understood, however, thatcryogenically cooled superconducting RF head coil array 10 may beimplemented with main magnet configurations other than a cylindrical,solenoid magnet that provides horizontal fields and/or, for example, maybe implemented with open magnet configurations, such as vertical magnetor a double-donut magnet. It is also understood that, according tovarious embodiments, main magnet 17 may be the main magnet of awhole-body scanner or may be the main magnet of a dedicated (e.g.,head-only) system (e.g., such as the main magnet described hereinbelowin connection with FIGS. 7-12).

FIG. 6 schematically depicts an illustrative RF head coil array thatincludes thermal radiation screening, in accordance with someembodiments of the present invention. More specifically, FIG. 6 depictsthe upper half of the coil depicted in FIG. 4A, further showing thermalradiation screens 17 that are used as an option to further protect thelow temperature of the RF coil 3 a and the non-metallic thermalconductor 5 a from heating by the radiation from the outer wall of thedouble-walled glass dewar and the environment outside the dewar. Thermalradiation screen 17 may be made from one or more materials, such asfoam, fabricate, cotton, or other non-metallic, good thermal insulationmaterials or combinations thereof.

As indicated above, while a superconductor RF head coil array inaccordance with the hereinabove embodiments may be implemented inconnection with a whole-body MRI scanner, such RF head coil arrays mayalternatively be used in dedicated, head-only MRI scanners. Inaccordance with some embodiments of the present invention, a dedicatedhead-only scanner may implement a superconductor main magnet inaccordance with embodiments represented by, and described in connectionwith, the following drawings. It will be understood, however, that MRIscanners employing a superconductor main magnet according to the ensuingembodiments may employ various RF coil configurations (e.g., array,non-array type, superconducting, non-superconducting, etc.), though someembodiments may employ superconducting RF head coil arrays implementedin accordance with embodiments described hereinabove.

FIG. 7 schematically depicts a cross-sectional view of a superconductingmain magnet of a head-only MRI system, the superconducting main magnetcomprising double-walled housing 41 and solenoid/helical coils 42, witha subject illustrated disposed therein with the subject's head arrangedwithin the diameter-sensitive volume 43 of the main magnet. As shown,double-walled housing 41 encloses a hermetically sealed region 47 thatis under at least a low vacuum condition, but preferably is under highvacuum (e.g., 10⁻⁶ to 10⁻¹² Torr), and also encloses an interior chamberregion 45 in which superconducting coils 42 are disposed and which isunder at least a low vacuum condition (e.g., 10⁻³ to 10⁻⁶ Torr).

More specifically, in accordance with some embodiments, thesuperconducting main magnet is an electromagnet system comprising avacuum thermal isolation housing 41 (e.g., a dewar) that is integratedwith a cryogenic system (not shown) to provide for coolingsuperconducting coils 42 via a heat pipe (not shown) and a heat sinkassembly (not shown) in thermal contact with the superconducting coils.Superconducting coils may be implemented as high temperaturesuperconductor (HTS) coils and, in some embodiments, may comprise atleast one of the following superconductor materials: YBaCuO, BiSrCaCuO,TIBiCaCuO, and MgB₂. By way of example, the temperature in the interiorchamber region in which the coils are disposed may be in the range ofabout 77K-80K.

In accordance with some embodiments, as shown, the coils are configuredas (i) a first coil set that is disposed in a first region to cover orsurround or otherwise be disposed adjacent to an individual's head, and(ii) a second coil set that is coaxial with the first coil set and isdisposed in a second region to cover or surround or otherwise bedisposed adjacent to the individuals shoulders or upper torso, whereinthe inner radius of the first set of coils is less than the inner radiusof the second set of coils, and the coils are configured to provide auniform magnetic field in the region of the individual's head. As willbe understood by those skilled in the art in view of the hereindisclosure, various embodiments may vary the number of coils per set,the coil radii, number of turns, longitudinal position and length, andelectric current magnitude and direction in each coil to provide adesired magnetic field distribution. In accordance with some embodimentsof the present invention, the longitudinal position and extension, thenumber of turns, and electric current direction of each coil aredesigned to provide 1-10 ppm uniform magnetic field within the firstregion for head imaging.

By way of example, the first set of coils may include at least two coilshaving an inner radius in a range of about 25-35 cm and disposed in afirst region of a length along the common axis in a range of 40-60 cm tocover a head and neck of a human body, and the second set of coils mayinclude at least one coil having an inner radius in a range of about30-40 cm and disposed in a second region of a length along the commonaxis in a range of 15-25 cm to cover a portion of a human torso. Invarious alternative embodiments, the length of the first and secondregions may, for example, range from about 20-70 cm and 10-40 cm,respectively, and the inner radii of the first and second set of coilsmay range from about 10-40 com and 20-50 cm, respectively. Someembodiments, may employ a length of the first and second regions in arange from about 10-20 cm and 20-30 cm respectively. Additionally, someembodiments may employ an inner radius of the first and second coils ofabout 10-20 cm and 20-30 cm, respectively.

By way of illustrative example, FIG. 8 depicts with reference to the z-rplane, with dimensions in meters (m), the longitudinal extent L2 of afirst set of coils (e.g., corresponding to the four leftmost coil setsdepicted in FIG. 7) having an inner radius of 0.28 meters, thelongitudinal extent L1 of a second coil set (e.g., corresponding to therightmost coil set in FIG. 7) having an inner radius of 0.38 meters, andDSV 43 having a radius that is about 0.1 meters and offset by about 0.05meters from the transition from the first to second set of coils (fromL2 to L1) along the z-axis, in accordance with an illustrative exampleaccording to some embodiments of the present invention.

FIG. 9 depicts a normalized current distribution for the main magnetcoil arrangement corresponding to the illustrative embodiment of FIGS. 7and 8. As shown, in accordance with some embodiments, at least one coilis wound to carry current in the reverse direction relative to othercoils.

FIG. 10 is an illustrative coil pattern (depicted in the z-r plane, withunits normalized to meters) of a 3 T head magnetic resonance imagingscanner, in accordance with various embodiments of the presentinvention. More specifically, active shield coil 51 is disposed at theouter side, main magnet coils 52 comprise eight coil sets, and adiameter-sensitive volume (DSV) 53 of homogeneous fields is about 200 mmin diameter (i.e., a radius of about 0.1 meter). The shield coil 51 mayhave a radius, for example, in the range of about 60-70 cm, though otherradii are possible depending on the particular implementation. By way ofillustrative, non-limiting example, the following table providesdimensions and current direction for coils arranged according to theembodiment of FIG. 10, wherein the first set of coils comprise coilnumbers 1 through 6, the second set of coils comprise coil numbers 7 and8, the shielding coil is identified as coil 9, R1 is the inner radius,R2 is the outer radius, Z1 is the first longitudinal position, Z2 is thesecond longitudinal position, and the current direction J is identifiedas positive (+) or negative (−):

Coil no. R₁ (m) R₂(m) Z₁(m) Z₂(m) J 1 0.2501 0.3028 −0.4132 −0.2897 + 20.2702 0.2916 −0.2519 −0.2325 + 3 0.2592 0.3033 −0.1873 −0.1327 + 40.2569 0.3032 −0.0765 −0.0349 + 5 0.2573 0.3027 0.0213 0.0606 + 6 0.26690.3012 0.1157 0.1680 + 7 0.3561 0.3821 0.1822 0.1980 − 8 0.3329 0.39290.2610 0.4433 + 9 0.6608 0.6615 −0.450 0.450 +

FIG. 11 is a plot showing the magnetic field distribution for theillustrative embodiment depicted in FIG. 10, with illustrativedimensions and current directions as per the foregoing table. As shown,a 3 T homogeneous field provides a 200 mm DSV.

FIG. 12 shows the fringe fields of one Gauss, three Gauss and five Gausslines for the field distribution of FIG. 11, in accordance with anillustrative embodiment of the present invention.

Accordingly, as may be appreciated, FIG. 10 illustrates a non-limitingexample of an embodiment according to the present invention. In thisexample, as described, the outer layer is an active shield coil 51, andthe depicted inner layer comprises main magnet coils 52 having eightcoil sets providing an asymmetric structure, with the coils on the righthand side (towards increasing z) having a bigger diameter foraccommodating a patient's shoulders. In this illustrative andnon-limiting embodiment, total length of the magnet is 0.86 m, the peakmagnetic field is 5.04 Tesla at a current density J=1.2×10⁸ A/m², andthe DSV 53 is 200 mm in diameter. According to these parameters, FIG. 11plots the field distribution in cylinder of z=−0.10□+−0.1 m, r=0.2 m. Incylinder of z=−0.10□+−0.1 m, r=0.1 m, the fringe fields of one Gauss,three Gauss and five Gauss lines is drawn in FIG. 12, and the 200 mm DSVis inside one Gauss line as expected and desired.

Accordingly, it may also be understood in view of the foregoing that fora head-only magnetic resonance imaging scanner according to embodimentsof the present invention, the bore surrounding a DSV 43 of homogeneousfields is preferably not much larger in diameter than what is necessaryto fit a patient's head, while the main magnet bore also includes aportion designed with a diameter having an appropriate size toaccommodate the shoulder as shown in FIG. 7. By contrast with awhole-body MRI, a head-only main magnet in accordance with someembodiments of the present invention has a smaller DSV, so the size ofsuperconducting magnet can be reduced, and a smaller Dewar and magnetsystem can be achieved, and the costs can be thus also be reduced.

The present invention has been illustrated and described with respect tospecific embodiments thereof, which embodiments are merely illustrativeof the principles of the invention and are not intended to be exclusiveor otherwise limiting embodiments. Accordingly, although the abovedescription of illustrative embodiments of the present invention, aswell as various illustrative modifications and features thereof,provides many specificities, these enabling details should not beconstrued as limiting the scope of the invention, and it will be readilyunderstood by those persons skilled in the art that the presentinvention is susceptible to many modifications, adaptations, variations,omissions, additions, and equivalent implementations without departingfrom this scope and without diminishing its attendant advantages. Forinstance, except to the extent necessary or inherent in the processesthemselves, no particular order to steps or stages of methods orprocesses described in this disclosure, including the figures, isimplied. In many cases the order of process steps may be varied, andvarious illustrative steps may be combined, altered, or omitted, withoutchanging the purpose, effect or import of the methods described. It isfurther noted that the terms and expressions have been used as terms ofdescription and not terms of limitation. There is no intention to usethe terms or expressions to exclude any equivalents of features shownand described or portions thereof. Additionally, the present inventionmay be practiced without necessarily providing one or more of theadvantages described herein or otherwise understood in view of thedisclosure and/or that may be realized in some embodiments thereof. Itis therefore intended that the present invention is not limited to thedisclosed embodiments but should be defined in accordance with theclaims that follow.

1. A superconducting radiofrequency coil array module configured forcryogenic cooling, comprising: a vacuum thermal isolation housingcomprising a double wall hermetically sealed jacket that (i) encloses ahermetically sealed interior space under a vacuum condition, and (ii)substantially encloses an interior chamber region that is separate fromthe hermetically sealed interior space and is configured to be evacuatedto a vacuum condition; a plurality of superconductor radiofrequencycoils disposed in said interior chamber region, each radiofrequency coilconfigured for at least one of generating and receiving a radiofrequencysignal for at least one of magnetic resonance imaging and magneticresonance spectroscopy; at least one thermal sink member disposed insaid interior chamber region and in thermal contact with thesuperconductor radiofrequency coils; and a port configured forcryogenically cooling at least the thermal sink member.
 2. The moduleaccording to claim 1, wherein the port is coupled to a cryocooler thatis thermally coupled to the at least one thermal sink member.
 3. Themodule according to claim 2, wherein the coupling of the cryocooler tothe port provides for sealing said interior chamber region such that theinterior chamber region is under a vacuum condition.
 4. The moduleaccording to claim 1, wherein said hermetically sealed jacket issealedly joined to a chamber having an interior space that iscoextensive with and is configured to be evacuated to substantially thesame vacuum condition as said interior chamber region, wherein said portis provided in said chamber.
 5. The module according to claim 4, whereinsaid chamber is configured as a double walled chamber that encloses ahermetically sealed intra-wall cavity that is under vacuum.
 6. Themodule according to claim 4, wherein the port is coupled to a cryocoolerthat is thermally coupled to the at least one the thermal sink member.7. The module according to claim 6, wherein the coupling of thecryocooler to the port provides for sealing said interior chamber regionsuch that the interior chamber region is under a vacuum condition. 8.The module according to claim 4, wherein the chamber is a double walledstainless steel chamber.
 9. The module according to claim 4, wherein thehermetically sealed interior space is under a vacuum condition having avacuum pressure in the range of about 10-6 to about 10-12 Torr, and theinterior chamber region is under a vacuum condition having a vacuumpressure in the a range of about 10-2 to about 10-6 Torr.
 10. The moduleaccording to claim 9, wherein said chamber is configured as a doublewalled chamber that encloses a hermetically sealed intra-wall cavitythat is under vacuum condition having a vacuum pressure in the range ofabout 10-6 to about 10-12 Torr.
 11. The module according to claim 1,wherein the hermetically sealed interior space is under a vacuumcondition having a vacuum pressure in the range of about 10-6 to about10-12 Torr, and the interior chamber region is under a vacuum conditionhaving a vacuum pressure in the a range of about 10-2 to about 10-6Torr.
 12. The module according to claim 1, wherein each radiofrequencycoil is in direct thermal contact with a respective one of said thermalsink members that are each in direct thermal contact with another ofsaid thermal sink members that is in thermal contact with saidcryocooler.
 13. The module according to claim 1, wherein saidradiofrequency coils comprise at least eight radiofrequency coils thatare azimuthally displaced about a common longitudinal axis at asubstantially common displacement along said longitudinal axis, and areconfigured for imaging a region surrounded by the radiofrequency coils.14. The module according to claim 1, wherein each of the radiofrequencycoils is configured to receive and not transmit radiofrequency signals.15. The module according to claim 1, wherein the superconductor materialcomprises an HTS material.
 16. The module according to claim 1, whereinthe vacuum thermal isolation housing and radiofrequency coils aredimensioned and configured for head imaging and not whole body imaging.17. The module according to claim 16, wherein said radiofrequency coilscomprise at least eight radiofrequency coils that are azimuthallydisplaced about a common longitudinal axis at a substantially commondisplacement along said longitudinal axis, and are configured forimaging a region surrounded by the radiofrequency coils.
 18. The moduleaccording to claim 1, wherein the radiofrequency coil array module isdimensioned and configured for use in a head-only magnetic resonanceimaging system that comprises a main electromagnet system comprising: afirst and second set of high temperature superconductor coils which areconfigured to be coaxial relative to a common longitudinal axis; whereinthe first coil set includes at least two coils having an inner radiusand disposed in a first region of a length along the common axis tocover a head and neck of a human body, and the second coil set includesat least one coil having an inner radius and disposed in a second regionof a length along the common axis to cover a portion of a human torso,wherein the inner radius of the second coil set is greater than theinner radius of the first coil set; and wherein the first and secondcoils are configured to provide a uniform magnetic field in the firstregion to provide for imaging a region of interest of the individual'shead when positioned within the first region.